THE ARTESIAN THERMAL GROUNDWATERS OF NORTHERN JORDAN - INSIGHTS INTO THEIR RECHARGE HISTORY AND AGE

The scarcity of water resources in Jordan imposes strategic difficulti es for economic development and has been a basis for disputes with nei ghbouring countries, A welcomed discovery in the past decade is the oc currence of artesian thermal groundwaters at Mukhebeh in the lower Yar mouk valley, which produce over 40 x 10(6) m(3) year(-1) of fresh wate r, The principal resources occur in the upper B2 Formation (Campanian) , a limestone overlain by confining marls of the B3 Formation (Maastri chtian), Additional resources are found near by along the eastern Jord an Rift Valley (JRV), in the underlying A7 limestone (Turonian).Ground waters in the Mukhebeh (B2) wells are low salinity (500 mg l(-1) TDS) Ca(Mg)-HCO3 waters with elevated HS- attributed to sulphate reduction driven by oxidation of aquifer-derived organic matter, The JRV (A7) gr oundwaters are similar to the B2 waters with the exception of higher M g2+ and HCO3- attributed to dolomite dissolution and sulphate reductio n. Groundwaters from JRV1 and the Maqla spring at Mukhebeh are NaCl wa ters with a strong Ca(Mg)-HCO3 facies (900 to 1000 mg l(-1) TDS), The elevated temperatures of all groundwaters (30 to 54 degrees C) can be attributed to heating along a local geothermal gradient of 2.5 degrees C per 100 m. Departures from this gradient are due to vertical flow i n fault zones and possibly to the local influence of an intrusion (MK- 7). All groundwaters are characterized by a late Holocene (modern) sta ble isotope signature. Groundwaters from the high yielding upper B2 aq uifer (MK wells) have a delta(18)O/delta(2)H composition demonstrating recharge in the B2 outcrop region, near and to the east of Irbid, wit hin the Yarmouk River basin. Conversely, the deeper A7 aquifer groundw aters (JRV wells) are recharged at a lower elevation in the rift valle y margin, where incised wadis have exposed these strata. The deepest A 1 to A6 strata (Maqla spring and JRV1 well) contain the isotopically m ost depleted groundwaters, Based on comparison with precipitation and phreatic groundwaters, recharge in the highest landscape of northern J ordan is likely, although a source of recharge from the northern Golan region of Syria cannot be excluded. All thermal waters are essentiall y H-3-free. Estimates of their mean subsurface residence times have be en constrained on the basis of C-14(DIC). Significant corrections have been made for C-14-dilution processes including carbonate dissolution , sulphate reduction and a tertiary process involving either incorpora tion of mantle CO2 or matrix exchange. All dilution processes were qua ntified on the basis of geochemical and C-13 mass balance equations. M odelled ages show that recharge for most B2 and A7 groundwaters occurr ed during the late Holocene (recent to 5 ka BP). (C) 1997 Elsevier Sci ence B.V.